Generally, steam as a working fluid in the steam turbine expands to high vacuum, so that it becomes wet steam containing water droplets over a saturated vapor line near an outlet of the steam turbine. In the steam turbine for thermal power generation, plural turbine stages including a low-pressure final turbine stage are operated by wet steam, and in the steam turbine for nuclear power generation or geothermal power generation, most of turbine stages are operated by wet steam.
When the steam turbine is operated by the wet steam, with the growth of water droplets generated in the turbine passage portion, there are problems related to performance and reliability which do not occur when the steam turbine is operated by the dry steam. The problems related to the performance include a moisture loss due to generation, growth and behavior of water droplets, and the problems related to the reliability include corrosion and the like caused by collision of water droplets against the rotor blades rotating at a high speed.
A ratio of the moisture loss to the total loss is high in the low-pressure turbine stage of the steam turbine for thermal power generation and in the multiple turbine stages of the steam turbine for nuclear power generation. Therefore, attempts have been made to calculate the moisture loss in the steam turbine.
As described above, the ratio of the moisture loss to the total loss is high, so that it is important to calculate the moisture loss with high accuracy from a view point of improving the performance of the steam turbine.